Atomic Structure, Bonding, Resonance, and Hybridization

1. Principle of Atomic Structure

Atoms are composed of three fundamental particles: protons, neutrons, and electrons. The arrangement and behavior of these particles determine the chemical properties of elements.

• Protons (p): Positively charged particles found in the nucleus.

• Neutrons (n): Neutral particles found in the nucleus.

• Electrons (e): Negatively charged particles found in orbitals around the nucleus.

• Electrons in shells determine reactivity.

• Orbitals: s, p, d, f — maximum 2 electrons per orbital.

2. Electronic Configuration

Electronic configuration describes the arrangement of electrons in an atom's orbitals.

• Order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s …

• Aufbau Principle: Electrons fill lowest energy orbitals first.

• Pauli Exclusion Principle: Maximum 2 electrons per orbital, with opposite spins.

• Hund's Rule: Electrons occupy degenerate orbitals singly before pairing.

3. Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a full set of eight valence electrons, resulting in greater stability.

• Atoms want 8 electrons for stability.

• Exceptions: H (2), B (6), P, S (can expand octet).

4. Common Bonding Patterns

Atoms form bonds to achieve stable electron configurations. The number of bonds and lone pairs varies by element.

• Carbon: 4 bonds, 0 lone pairs

• Nitrogen: 3 bonds, 1 lone pair

• Oxygen: 2 bonds, 2 lone pairs

• Hydrogen: 1 bond, 0 lone pairs

• Halogens: 1 bond, 3 lone pairs

General Rule: More formal charge = less stable.

5. Resonance

Resonance structures are different Lewis structures for the same molecule, showing delocalization of electrons.

• Only electrons move, not atoms.

• Equivalent forms contribute equally to the resonance hybrid.

• Best resonance structure = full octets + minimal charge.

6. Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals for bonding.

• sp3: 4 sigma bonds, 109.5° geometry (tetrahedral)

• sp2: 3 sigma bonds, 120° geometry (trigonal planar)

• sp: 2 sigma bonds, 180° geometry (linear)

• More s-character = more acidic (sp > sp2 > sp3).

7. Molecular Orbitals

Molecular orbital theory explains bonding by combining atomic orbitals to form molecular orbitals.

• Sigma (σ) bond: Strongest type of covalent bond.

• Pi (π) bond: Weaker than sigma bonds; formed by side-to-side overlap.

• More electrons in low-energy orbitals = more stable molecule.

• Antibonding orbitals (σ*, π*) are destabilizing.

Dipole Moment, Resonance & Acidity/Basicity

1. Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges in a molecule, indicating molecular polarity.

• Dipole: Difference in electronegativity between atoms creates a dipole.

• Polar molecules: Have a net dipole moment.

• Nonpolar molecules: Symmetrical structure cancels dipoles.

2. Resonance & Acidity/Basicity

Resonance stabilization affects the strength of acids and bases.

• If the conjugate base is resonance-stabilized, the acid is stronger.

• Stronger acid = lower pKa

Factors affecting acidity:

• Charge

• Electronegativity

• Atom size

• Resonance

• Inductive effect (halogens increase acidity when near acidic H)

Alkanes, Nomenclature & Conformations

1. Nomenclature of Alkanes

Alkanes are saturated hydrocarbons with only single bonds. Their names are based on the number of carbon atoms and the presence of substituents.

• Find the longest carbon chain.

• Number the chain to give substituents the lowest possible numbers.

• Prefix: number + substituent.

• Alphabetical order for substituents.

2. Conformations & Newman Projections

Alkanes can rotate around single bonds, leading to different spatial arrangements (conformations).

• Staggered: Most stable conformation.

• Eclipsed: Least stable conformation.

• Ranking: Anti > Gauche > Eclipsed (in order of decreasing stability).

Newman projections: Visualize groups around a bond.

• Anti: Large groups opposite each other (most stable).

• Gauche: Large groups adjacent.

• Eclipsed: Least stable when large groups overlap.

Radicals

1. Radical Formation and Stability

Radicals are atoms or molecules with an unpaired electron, making them highly reactive.

• Mechanism steps:

  1. Initiation: Radicals are formed.

  2. Propagation: Radicals react and create new radicals.

  3. Termination: Two radicals combine.

• Radical stability: 3° > 2° > 1° (tertiary > secondary > primary).

Chirality & Optical Activity

1. Chirality

A chiral center is a carbon atom bonded to four different groups, resulting in non-superimposable mirror images (enantiomers).

• Chiral center = carbon with 4 different groups.

• Enantiomers = non-superimposable mirror images.

• Diastereomers = stereoisomers that are not mirror images.

2. Optical Rotation

Chiral compounds rotate plane-polarized light in opposite directions.

• (+) or (d): Clockwise rotation.

• (-) or (l): Counterclockwise rotation.

• Important: R/S does not correlate with (+)/(-).

Allylic Bromination, SN1 & SN2

1. Allylic Bromination / Halogenation

Bromination occurs at the allylic position (next to a double bond), often favored by resonance stabilization. A typical reagent is NBS (N-bromosuccinimide).

2. Nucleophilic Substitution: SN2 Mechanism

SN2 (bimolecular nucleophilic substitution) is a one-step reaction where the nucleophile attacks the substrate from the opposite side, leading to inversion of configuration.

• One step

• Backside attack

• Inversion of configuration

• Favored by:

  • Primary substrate

  • Strong nucleophile

  • Polar aprotic solvent

3. Nucleophilic Substitution: SN1 Mechanism

SN1 (unimolecular nucleophilic substitution) is a two-step reaction involving formation of a carbocation intermediate, followed by nucleophilic attack.

• Two steps

• Carbocation intermediate

• Racemization (loss of stereochemistry)

• Favored by:

  • Tertiary substrate

  • Weak nucleophile

  • Polar protic solvent

Summary Table: Common Bonding Patterns

Key Equations

• Dipole Moment: where is the dipole moment, is the charge, and is the distance between charges.

• Formal Charge:

Additional info: Some explanations and context have been expanded for clarity and completeness, especially for resonance, hybridization, and reaction mechanisms.